I think most people who've been excited by the slogan "America First"--including Donald Trump--would be unable consciously to say why that slogan appealed to them.
It's easy to say, "My God, they're all so ignorant.""They know nothing of history!" That's at least reassuring. But I don't think it's how it works.
(Americans don't however know what "Je suis partout" was, not even at a half-conscious level, although probably many would sense that the slogan--"I am everywhere"--has an unpleasant resonance.
No one in his right mind wants to remember what that was really about. But people who aren't in their right minds will always be drawn to it.